1. Field of the Invention
The present invention relates to a backlight assembly, and more particularly, a backlight assembly including a plurality of light emitting diodes (LEDs) and a liquid crystal display (LCD) module using the backlight assembly.
2. Discussion of the Related Art
Flat panel display (FPD) devices that have a relatively light weight, a thin profile, and low power consumption characteristics are being developed and commonly used as a substitute for cathode ray tube (CRT) devices. Generally, display devices are classified according to their ability for self-emission as emissive display devices and non-emissive display devices. Emissive display devices display images by taking advantage of their ability to self-emit light. By contrast, the non-emissive display devices require a light source since they do not emit light by themselves. For example, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescent display (ELD) devices are commonly classified as emissive display devices. Liquid crystal display (LCD) devices are classified as non-emissive display devices. LCDs are commonly used in notebook and desktop computers because of their high resolution, ability to display colored images, and high quality image display.
An LCD module includes an LCD panel for displaying images and a backlight unit for supplying light to the LCD panel. The LCD panel typically includes two substrates facing and spaced apart from each other, and a liquid crystal material interposed therebetween. Liquid crystal molecules of the liquid crystal material have a dielectric constant and refractive index anisotropic characteristics due to their long, thin shape. In addition, two electric field generating electrodes are formed on the two substrates, respectively. Accordingly, an orientation alignment of the liquid crystal molecules can be controlled by supplying a voltage to the two electrodes to change transmittance of the LCD panel according to polarization properties of the liquid crystal material. However, since the LCD panel is a non-emissive-type display device, an additional light source is required.
Typically, a backlight unit is disposed under the LCD panel in order for the LCD device to display images using light produced by the backlight unit. In general, backlight units may be classified into two types according to a placement of the light source, such as an edge-type backlight unit and a direct-type backlight unit. As display areas of the LCD devices become larger, direct-type backlight units that include a plurality of light sources have become more commonly used in order to provide increased brightness.
Generally, discharge lamps, such as cold cathode fluorescent lamps (CCFLs) or external electrode fluorescent lamps (EEFLs), are used as a light source for the backlight unit. More recently, light emitting diodes (LEDs) have gradually been used more often as the light source of the backlight unit to improve color reproducibility and increase brightness of the display.
FIG. 1 is an exploded view of an LCD module with a backlight assembly according to the related art. An LCD panel 10 and a backlight unit 20 are assembled with mechanical structural elements that protect these components and to prevent light leakage from the assembled LCD module 1. Generally, the LCD panel 10, the backlight unit 20, and several mechanical elements taken together may be referred to as an LCD module 1.
The LCD module 1 includes an LCD panel 10, a backlight unit 20 located under the LCD panel 10, and a main frame 40, which is preferably square or rectangular and surrounds the edges of the LCD panel 10 and the backlight unit 20. Additionally, a bottom frame 50 is provided on a bottom surface of the backlight unit 20 to provide structural support and to prevent light leakage from the unit when combined with the main frame 40. Furthermore, a top frame 60 surrounds a front edge of the LCD panel 10 such that the LCD module 1 is complete when the main frame 40, the bottom frame 50, and the top frame 60 are combined.
In addition, the backlight unit 20 further includes a plurality of printed circuit boards (PCBs) 22 that are disposed along an inner surface of the bottom frame 50, a plurality of light emitting diodes (LEDs) 24 packaged on the PCBs 22, a reflective sheet 26 having a plurality of through holes 28 corresponding to the plurality of LEDs 24 and contacting the PCBs 22 and the bottom frame 50 except at the LEDs 24, and a plurality of optical sheets 32 covering the plurality of LEDs 24.
Meanwhile, light from the plurality of LEDs 24 is reflected by the reflective sheet 26, and then the reflected light is altered through the plurality of optical sheets 32. The altered light is transferred to the LCD panel 10, which allows the LCD panel 10 to display a bright image. For example, in order to emit a white light, the plurality of the LEDs 24 may be driven by mixing light from red (R), green (G), and blue (B) LEDs emitting red, green, and blue colors, respectively, arranged in a predetermined order. Although not shown, the optical sheet 32 may include a diffuser sheet and a prism sheet.
FIG. 2 is a schematic cross-sectional view taken along line “II-II” of FIG. 1. As shown in FIG. 2, in the backlight assembly 20, light (not shown) from the LEDs 24, i.e., light directly emitted and reflected by the reflective sheet 26, is passed through the optical sheet 32 and enters the liquid crystal panel 10 (of FIG. 1) as a light source.
This backlight assembly 20 has several disadvantages. Generally, the LEDs 24 are arranged in a row on each of the PCBs 22. Accordingly, when some of the LEDs 24 become damaged, the whole PCB 22 having the damaged LEDs 24 must be replaced regardless of the number of the LEDs 24 actually damaged. In some cases, during the repair of damaged PCBs 22, normally functioning LEDs 24 may become damaged. Therefore, according to the backlight assembly of the related art, cost and time for repairing the backlight assembly may be increased.
Further, it is impossible to change the positions of the LEDs 24 once they are set on the PCBs 22. Consequently, the user cannot replace the LEDs 24 with different LEDs if the brightness of the backlight assembly 20 is not satisfactory.
Additionally, in order to have significantly high brightness and contrast in a specific portion of the liquid crystal panel 10 (of FIG. 1), such as an explosion scene in a movie, for example, the backlight assembly 20 should be driven by a division driving method. However, it is difficult to drive the backlight assembly 20 according to the related art using the division driving method.
Moreover, the backlight assembly 20 should have enough space between the LEDs 24 and the optical sheet 32 to facilitate adequate color mixing and to increase the divergence angle (not shown) of the LEDs 24. Further, various diffusing elements, such as a diffuser member (not shown), may be interposed between the LEDs 24 and the optical sheet 32 to solve some of the problems of the related art. However, even if the backlight assembly 20 including the diffuser member for selected LEDs 24 is driven by the division driving method the backlight assembly 20 still suffers from weak brightness at the boundary.